Flexible array member (c99) inside a structure

Question

I\'ve being using this code a while now, and it works fine, but it gave me some headache to implement it. It uses Flexible Array Member (FAM) aka Struct Hack. Now that C99 has t

Answer 1

The C name for that construction is "flexible array member" (it might help your searches), or "struct hack" before C99.

Look at 6.7.2.1 in the Standard; there is example usage in the text.

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You may also be interested in "Variable Length Arrays". See 6.7.5.2 in The Standard.

Answer 2

Actually, it's not variable-length arrays that you want to use here, but the struct hack, aka "incomplete types", aka the "flexible array member":

typedef struct nO
{
    int oper;
    int nops;
    struct nO *ptn[];  // <== look ma, no index!
} nodoOper;

// skip a bit
// no more (n-1) --------\
tam = sizeof(nodoOper) + n * sizeof(nodoOper *);

Only the last member of a struct may be "flexible".

Variable-length arrays are a different feature:

void foo(int size)
{
    float a[size];               // run-time stack allocation
    printf("%lu\n", sizeof(a));  // and run-time sizeof (yuck)
}

(Oh, and these things are called arrays, not matrices.)